1. Field of the Invention
The present invention relates to an optical receiver, in particular, the invention relates to an optical receiver with a function to generate the LOS alarm reliably.
2. Related Prior Art
Conventional optical receiver applicable to the optical communication system has generally provided the function to generate the LOS (Loss Of Signal) alarm, in which, the optical receiver asserts an alarm when the optical input signal losses. The United States Patents, US 2007/0036541A and US 2007/0104494A, have disclosed an optical receiver with a function to detect the input optical signal at a downstream of the coupling capacitor set between the receiver optical module and the main amplifier. The optical receiver disclosed therein may prevent the mis-negation of the LOS alarm due to the secondary or more order time lag of the input optical signal. A Japanese Patent Application published as JP-2003-152460A has disclosed another optical receiver in which the optical receiver generates an alarm even when the DC optical signal with a substantial magnitude is received. Still another Japanese Patent published as JP-H10-243030A has disclosed a technique that the optical receiver detects the peak level of the clock signal that is extracted from the input optical signal, and saturates the signal path when the peak level thus detected becomes less than a preset level.
The optical receiver disclosed in the United States Patent application requires an additional circuits, for instance, a peak-holding, a bottom-holding, a digital filter and so on, in the downstream of the optical receiver. Moreover, the optical receiver is necessary to detect the signal level in the primary path before the coupling capacitor where the signal is sent by the differential mode, which is hard to apply the small sized receiver optical module where the count of the lead pins is severely restricted. An ordinary package used in the small sized optical module has five lead pins, each for the power supply, the ground, the bias for the photodiode (hereafter denoted as PD), and for the signal output with the differential mode. The optical receiver disclosed in the prior United States Patent listed above is necessary to provide additional two lead pins for the LOS alarm and for the reference for the loss level.
The optical receiver in the Japanese Patent, JP-H10-243030A, provides the squelching function but the function therein is controlled by the output of the trans-impedance amplifier (hereafter denoted as TIA) coupled in the AC coupling mode; accordingly, the comparator that decides the absence of the input optical signal is easily to respond a noise or a fluctuation appeared in the output of the TIA, which causes the mis-assertion and the mis-negation of the LOS alarm.